1. Technical Field
The present disclosure relates to magneto-rheological fluids and clutches using the magneto-rheological fluids, and more particularly to a magneto-rheological fluid for use at high shearing speed and a clutch using the magneto-rheological fluid.
2. Related Art
A magneto-rheological (MR) fluid is a type of fluid in which magnetic particles of, e.g., iron (Fe) are dispersed in a dispersion medium such as oil. Under no influence of a magnetic field, magnetic particles are suspended randomly in the dispersion medium in the MR fluid. In the presence of an externally applied magnetic field to the MR fluid, the magnetic particles form a large number of clusters along the direction of the magnetic field, and the yield stress increases. Thus, the MR fluid is a material whose rheological properties or mechanical properties can be easily controlled by using an electrical signal, and thus, application of the MR fluid to various fields has been studied. The MR fluid is currently principally used for direct drive devices such as shock absorbers for automobiles and seat dampers for construction machinery.
Magnetic particles typically used in an MR fluid have an average particle size of several micrometers to several tens of micrometers. A magnetic fluid is another type of fluid in which magnetic particles are dispersed in a dispersion medium such as oil, as well as an MR fluid. Magnetic particles for use in the magnetic fluid have particle sizes of about several nanometers to 10 nm, and are caused to vibrate by Brownian motion resulting from thermal energy. Thus, magnetic particles do not form clusters even in the presence of an applied magnetic field to the magnetic fluid and the yield stress does not increase. In this aspect, the magnetic fluid completely differs from the MR fluid. The MR fluid uses particles larger than those used in the magnetic fluid, in order to form clusters. Thus, when the MR fluid is left untreated, caking occurs due to sedimentation of magnetic particles. In addition, repetitive application and cancel of a magnetic field causes secondary agglomeration of magnetic particles, resulting in failure of maintenance of a stable dispersion state.
As a technique for enhancing stability of an MR fluid, inventors of the present disclosure filed a patent application (e.g., Japanese Patent Publication No. 2009-117797) for an MR fluid including nano-sized magnetic particles whose average particle size ranges several tens of nanometers to several hundred of nanometers. The nano-size of magnetic particles reduces sedimentation and secondary agglomeration of the magnetic particles.